The invention relates to a process for making paper and to the use of starch in said process.
In order to increase the strength properties of paper, it is common practice to add cationic starch at the wet-end stage of the papermaking process. The wet-end of the papermaking process refers to the stages of the papermaking process, wherein a pulp of fibers obtained from cellulose-based materials, such as recycled, used paper, wood, cotton, or alternative sources, is being processed. The term xe2x80x9cwet-endxe2x80x9d originates in the large amounts of water, in the presence of which the pulp is processed.
Recently, there have been several, trends in the papermaking industry, which call for more starch in the paper. One of these trends is the environmental demand to recycle paper. As paper is recycled, the fibers of the paper tend to become shorter and weaker, the latter of which is due to reduced interactions among the fibers. As a result, increased amounts of starch are necessary in the wet-end of the papermaking in order to produce a paper which is sufficiently strong. It has been found that after paper has been recycled a certain number of times, the loss of strength due to recycling cannot always be compensated by simply adding starch. Accordingly, recycling ultimately leads to paper having an inferior paper strength.
Another trend is the urge to produce cheaper paper. This can be achieved by incorporating large amounts of a cheap filler into the paper. However, a larger filler content of the paper results in a deterioration of paper strength, which gives rise to a demand for the addition of increased amounts of starch in the wet-end.
Yet another trend concerns a change in the apparatuses used in the papermaking process. The conventionally used size-press is more and more being replaced by a premetering size-press. The use of a premetering size-press often has the effect that starch penetrates to a lesser degree into a paper sheet than when a conventional size-press is used. As a result, the starch provides a smaller contribution to the strength of the paper. Moreover, the use of a premetering-size press for pigmentizing diminishes the internal strength of the paper even more. Therefore, it is desired to incorporate higher amounts of starch in the paper, leading to an increase of the strength of the paper. In this regard, it is particularly important that, when increased amounts of starch are added in the wet-end of the papermaking process, a high retention of said starch is obtained. In other words, it is important that the high amounts of starch are actually incorporated into the paper, and are not lost in the process water of the pulp.
In xe2x80x9cAnionic starch: an effective wet-end concept for enhancing paper strengthxe2x80x9d, Proceedings of the PITA Annual Conference, 87-91, Manchester, October 1997, J. Terpstra and R. P. Versluijs have proposed to use anionic starch instead of cationic starch as a strengthening agent in the wet-end of the papermaking process, in order to achieve a greater internal strength of the paper produced. This concept of using anionic starch has also been described in P. H. Brouwer, Wochenblatt fxc3xcr Papierfabrikacion, 19 (1997), 928-937 WO-A-93/01353 WO-A-96/05373, and may be explained as follows.
The fibers and filler particles, which are used to produce paper from, are negatively charged. When cationic starch is used as a paper strengthening agent, its retention is mainly caused by the interaction between the positively charged starch and the negatively charged fibers and filler particles. In order to adhere anionic starch molecules onto anionic fibers and filler particles, use is made of a so-called cationic fixative. In principle, any cationic paper aid can be used as a fixative for the anionic starch, although some lead to better results than others. Because they are cheap and hardly affected by water hardness, polyaluminum chlorides are considered very attractive fixatives. Other materials that have been proposed for use as a fixative in this regard are, inter alia, alum, or cationic polymers, such as polydimethyldiallylammonium chloride and polyamines.
It has been found that, by using anionic starch in combination with a suitable fixative, it is possible to incorporate up to five times as much starch into a paper sheet in comparison with the case wherein only cationic starch is used as a strengthening agent. Of course, this results in a much stronger paper sheet. At the same time, the retention of the starch in a papermaking process is much higher when anionic starch and a fixative are used instead of cationic starch.
A disadvantage of the use of anionic starch instead of cationic starch in the wet-end of the papermaking process resides in the necessity of using a fixative. Even though some of the fixatives proposed in the art are relatively cheap, the costs of the paper that is produced may increase considerably because of the use of the fixative. Also, as the fixative is a cationic compound, it is inevitable that anionic counterions are added to the paper along with the fixative. Often, the counterions are chloride ions which are corrosive. Furthermore, the use of a fixative may lead to a hardening of the process water and to the production of salts, which may interfere with other papermaking aids.
Accordingly, a need exists for the provision of an alternative manner of increasing the amount of starch that can be incorporated into paper by addition thereof in the wet-end of the papermaking process. This alternative manner should preferably not have the disadvantages discussed above. It is particularly desired that the use of a fixative may be omitted.
Surprisingly, it has now been found that the above goals can be reached by using a specific type of cross-linked starch in the wet-end of the papermaking process. Thus, the invention relates to a process for making paper, wherein in the wet-end a cross-linked starch is added to an aqueous suspension of cellulose-based fibers, which starch has a degree of cross-linking, measured as a FANN viscosity at 300 rpm, of less than 50 mPas, preferably less than 25 mPas, and a FANN-A value of less than 2.5, preferably less than 0.5.
It is a great advantage of the present invention, that the means are provided for incorporating amounts of starch into paper by addition of starch in the wet-end of the papermaking process, which were hitherto not possible without the use of additives. Further, the retention of the starch added in the wet-end of a papermaking process according to the invention is significantly increased in comparison with the prior art processes. Furthermore, in case the added starch is an anionic starch, it has been found that it is not necessary to add a fixative